Magnetic characteristics of recording magnetic tapes differ with kinds of tapes, such as normal tapes and chrome tapes, because of different magnetic materials used for tapes. Therefore, the bias level of the recording head has to be set in accordance with the kind of the recording tape for magnetically biasing the magnetic tape. Meanwhile, during reproducing the most suitable equalizer characteristic has to be set so as to restore the recorded signal properly. Some conventional tape recorders have a bias level selector switch and an equalizer characteristic selector switch for respectively setting the most suitable bias level for the recording head during recording and most suitable equalizer characteristic for the reproducing amplifier during reproducing.
Although these switches had to be manually operated, the inventors of the present invention had recently proposed, in a Japanese Patent application No. (53(1978)-35485) an automatic bias level setting circuit which automatically sets the most suitable bias level by means of an examination of the magnetic characteristic of the magnetic tape using a test bias signal.
Although the level of a bias signal is suitably set either manually or automatically to reduce distortion of the recording and reproducing signals, when the sensitivity of magnetic tapes differ from tape to tape of the same kind, the reproduced signals have variation in level. Here, the word sensitivity means the recording efficiency of a magnetic recording tape. This variation in sensitivity of recording tapes causes a tape recorder to erroneously reproduce the original recording signal so that high-fidelity reproduction is not achieved.
Especially when a compression-expansion type noise reduction method, such as Dolby noise reduction technique, is adopted, the dynamic range compression characteristic in recording and the dynamic range expansion characteristic in reproducing do not correspond to each other because of the deviation of the level of reproducing signal from the level of the recording signal inasmuch as the compression and expansion degrees in such noise reduction technique are determined by the level of the signal and the frequency thereof. Consequently, the degree of dynamic range expansion during reproducing differs from the degree of dynamic range compression during recording and therefore, faithfull recording and reproducing of an audio signal cannot be performed.